1. Field of the Invention
The present invention relates generally to laser treatment of soft tissue, and more particularly to photoselective vaporization of the prostate PVP, and to photoselective vaporization of other tissue.
2. Description of Related Art
Benign Prostatic Hyperplasia (BPH) is a condition wherein continued growth of the prostate restricts the passage of urine through the lower portion of the bladder and the urethra. BPH is often treated by surgically removing excess prostate tissue from the transitional zone of the prostate that is pressing on the urethra, which usually relieves the bladder outlet obstruction and incomplete emptying of the bladder caused by the BPH.
Recently, the most commonly employed procedure for removal of excess prostate tissue has been transurethral resection of the prostate, also known as TURP. In the TURP procedure, the surgeon utilizes a standard electrical cutting loop to shave off small pieces of the targeted tissue from the interior of the prostate. At the end of the operation, pieces of excised prostate tissue are flushed out of the bladder using an irrigant.
While effective, the TURP procedure is known to cause numerous side effects, including incontinence, impotence, retrograde ejaculation, prolonged bleeding and TUR syndrome. Recently, alternative procedures have been developed which reduce or avoid the side effects associated with TURP. One class of procedures involves “cooking” prostate tissue by heating it to a to a temperature above 45 degrees Celsius, causing tissue coagulation. Typically this is accomplished using electrically resistive elements such as: radio frequency (RF), microwave, or long-wavelength lasers. An example of a procedure of this nature is discussed in U.S. Pat. No. 6,064,914 by Trachtenberg (“Thermotherapy Method”). Because these procedures leave the thermally-treated tissue in place, post-procedure edema, dysuria, and retention rates are relatively high. Further, use of thermal procedures requires the patient to be catheterized for several days following the procedure, and may cause extensive and unpredictable scarring of the intra prostatic urethra.
Another class of procedures involves vaporizing or ablating the targeted tissue using laser light. These procedures generally avoid the high infection rates and scarring problems of thermally-based procedures. However, laser ablation of prostate tissue has to date, required the use of an expensive laser capable of generating high-power laser light. The high cost of purchasing or leasing such a laser results in a concomitant increase in the cost of the procedure. Finally, the ablation process typically occurs slowly, resulting in a lengthy procedure time.
The Ho:YAG laser and its fiberoptic delivery system is an example of a laser that is commonly used for ablating prostate tissue. The Ho:YAG laser generates pulses of 2100 nm light that are strongly absorbed by water in the prostate tissue and in the saline irrigant positioned between the distal end if the fiberoptic and the tissue. The absorption coefficient of water is so high at 2100 nm that 50% of the light is absorbed within 0.2 mm of water. Consequently even a thin layer of irrigant positioned between the distal end on the fiberoptic and the tissue will absorb a large fraction of the laser light. Furthermore with the short pulse durations (Tp<0.5 ms) and large pulse energies (Ep>1.0 joule) used for ablating prostate tissue the irrigant is explosively boiled creating a shock wave that tears tissue. Because water is such a large constituent of prostate tissue and blood, there is essentially no selective absorption by blood. This combination of violent tissue disruption and the superficial unselective light penetration leads to poor hemostasis.
Nd:YAG lasers operating at 1064 nm have also been used for ablating prostate tissue. Although 1064 nm light is hemostatic at high power levels its low absorption in blood and prostate tissue leads to inefficient ablation and a large residual layer of thermally denatured tissue several millimeters thick. After surgery, the coagulated, thermally denatured tissue swells and leads to transient urinary retention, which can cause long catheterization times, painful urination, and high infection rates.
Frequency doubled Nd:YAG lasers operating at 532 nm in a Quasi continuous mode at power levels up to 60 watts have been used to efficiently and hemostatically ablate prostate tissue. These lasers are pumped by CW krypton arc lamps and produce a constant train of Q-switched pulses at 25 kHz. The high Q-Switch frequency makes the tissue effects indistinguishable from CW lasers of the same average power. The 532 nm light from these lasers is selectively absorbed by blood leading to good hemostasis. When ablative power densities are used, a superficial layer of denatured prostate tissue less than 1 mm is left behind. This thin layer of denatured tissue is thin enough that the immediate post surgical swelling associated with other treatment modalities is greatly reduced. This reduced swelling leads to short catheterization times and less dysuria. At high powers, 532 nm lasers induce a superficial char layer (an absorptive, denatured layer) that strongly absorbs the laser light and greatly improves the ablation efficiency. The problem with the existing 532 nm lasers used to date is that they are large, expensive, inefficient, and have a highly multi-mode output beam that makes them inefficient for ablating prostate tissue. Furthermore, residual coagulation of tissue due to the procedure remains significant using the techniques known in the prior art, as discussed below.
High power densities are required for rapid and efficient vaporization of prostate tissue. The difficulty of achieving higher average output power densities is that when high input powers are supplied to the laser element from an excitation source such as an arclamp a large amount of heat is generated in the lasing element. This heat induces various deleterious effects in the lasing element. In particular the temperature difference between the coolant and the hot lasing element generates a thermally induced graded index lens that decreases the beam quality of the laser and causes the laser to operate with more transverse optical modes than it would otherwise.
The M2 parameter is a well established convention for defining the beam quality of a laser and is discussed in pages 480-482 of Orazio Svelto and David C. Hanna, Principles of Lasers, Plenum Press, New York, 1998, which is incorporated herein by reference. The beam quality measures the degree to which the intensity distribution is Guassian. The quantity M2 is sometimes called inverse beam quality rather than beam quality but in this application it will be referred to as beam quality. M2 is defined as                     M        x        2            ≡                                    (                                          σ                x                            ⁢                              σ                f                                      )                    NG                                      (                                          σ                x                            ⁢                              σ                f                                      )                    G                      =          4      ⁢                        π          ⁡                      (                                          σ                x                            ⁢                              σ                f                                      )                          NG              ,
where π refers to the number 3.14 . . . , σ is used to represent the spot size, the subscripts x and f represent the spatial and frequency domains along the x-axis, respectively, and the subscripts G and NG signify Guassian and non-Guassian, respectively. The x-axis is transverse to the direction of propagation of the beam. The beam quality in any direction transverse to the beam may be essentially the same. Therefore the subscript x is dropped from the M2 elsewhere in the specification. The beam widths or σs are determined based on the standard deviation of the position, where the squared deviation of each position is weighted by the intensity at that point. The beam width in the frequency domain σf is the beam width of the beam after being Fourier transformed.
The formula usually used for calculating the angular divergence, θ, of a beam of light of wavelength λ is strictly valid only for a beam having a Guassian intensity distribution. The concept of beam quality facilitates the derivation of the angular divergence, θ, for the beam with a non-Guassian intensity distribution, according to   θ  =                    M        2            ⁡              (                              2            ⁢            λ                                π            ⁢                                                   ⁢                          σ              x                                      )              .  
For example, a TEM00 laser beam has a high beam quality with an M2 of 1, whereas by comparison, high power surgical lasers operate with M2 values greater than 100.
The Applicants have recognized that high power lasers typically have an M2>144. The larger number of modes makes M2 larger and makes it difficult to focus the light into small, low numerical aperture fibers and reduces the ability to project high power density light onto tissue. As a result, the vaporization efficiency of CW arclamp pumped 532 nm lasers on prostate tissue is significantly reduced.
Other aspects and advantages of the present invention can be seen on review of the drawings, the detailed description and the claims, which follow.